Time difference of arrival (TDOA) can be used to determine the location of an object relative to a tablet or other computing device. For example, a pen (e.g., a stylus) may include a transmitter configured to transmit an ultrasound signal. The ultrasound signal may be detected by receivers (e.g., microphones) on the tablet. Timing differences between signals received at different receivers can be used to detect position and movement of the pen (based on how long it took the ultrasound signal to travel from the pen to the microphones).
Ultrasound signal energy decays faster than audio band signal energy and may require line of sight (LOS) between transmitters and receivers. Systems that use an ultrasound transmitter and at least one receiver may not properly determine the location of the pen when a LOS signal path between the transmitter and the at least one receiver is blocked. For example, when a user is using the pen, his or her hand or another object can block the signal path between the pen and a receiver. One approach to avoid the blocking problem is to have many redundant receivers operating at the same time. This approach uses more power due to having all receivers (e.g., microphones) powered at the same time and consumes more bandwidth between a hardware codec and a system processor.